Empirical Formula for Molecular Formula

Empirical Formula and Molecular Formula

An empirical formula represents the simplest whole-number ratio of various atoms present in a compound. A molecular formula shows the exact number of different types of atoms present in a molecule of a compound. For example, the empirical formula of hydrogen peroxide is HO, while its molecular formula is $H_2O_2$ — the molecular formula is always a whole-number multiple of the empirical formula.

If the mass per cent of the various elements present in a compound is known, its empirical formula can be determined; the molecular formula can further be obtained if the molar mass is also known.

From Mass Per Cent to Empirical Formula

Step 1 — Convert the mass per cent to grams. Since mass per cent is given, it is convenient to use 100 g of the compound as the starting material: each percentage then converts directly into a mass in grams.

Step 2 — Convert into number of moles of each element. Divide each element's mass by its atomic mass. This gives the number of moles of each constituent element in the 100 g sample.

Step 3 — Divide each mole value by the smallest value among them. This gives the simplest ratio. In case the ratios are not whole numbers, they may be converted into whole numbers by multiplying by a suitable coefficient.

Step 4 — Write the empirical formula by mentioning the numbers after writing the symbols of the respective elements.

From Empirical Formula to Molecular Formula

Step 5 — (a) Determine the empirical formula mass by adding the atomic masses of the atoms in the empirical formula. (b) Divide the molar mass by the empirical formula mass:

$n = \frac{\text{Molar mass}}{\text{Empirical formula mass}}$ …